Ultrasensitive fluorescent ratio imaging probe for the detection of glutathione ultratrace change in mitochondria of cancer cells

A ratiometric fluorescent dark box and smartphone integrated portable sensing platform based on hydrogen bonding induction for on-site determination of enrofloxacin

Enrofloxacin (ENR) residues in animal-derived food and water threaten human health. Simple, low-cost and on-site detection methods are urgently needed. Blue emitting carbon quantum dots (CQDs) and orange rhodamine B (RhB) were used as recognition and reference signals, respectively, to construct a ratiometric fluorescence sensor. After the addition of ENR, the color of the sensor changed from orange to blue because hydrogen bonding induced a considerable increase in CQDs fluorescence. Based on this mechanism, a simple and low cost on-site portable sensing platform was constructed, which integrated a stable UV light strip and a smartphone with voice-controlled phototaking function and an RGB app. The t-test results of spiked ENR recoveries for diluted milk, honey and drinking water revealed no significant differences between the ratiometric fluorescent sensor and portable sensing platform. Thus, this portable sensing platform provides a novel strategy for on-site quantification of quinolone antibiotics in foodstuffs and environmental water.

Advances in small-molecule fluorescent probes for the study of apoptosis

Apoptosis, as type I cell death, is an active death process strictly controlled by multiple genes, and plays a significant role in regulating various activities. Mounting research indicates that the unique modality of cell apoptosis is directly or indirectly related to different diseases including cancer, autoimmune diseases, viral diseases, neurodegenerative diseases, etc. However, the underlying mechanisms of cell apoptosis are complicated and not fully clarified yet, possibly due to the lack of effective chemical tools for the nondestructive and real-time visualization of apoptosis in complex biological systems. In the past 15 years, various small-molecule fluorescent probes (SMFPs) for imaging apoptosis in vitro and in vivo have attracted broad interest in related disease diagnostics and therapeutics. In this review, we aim to highlight the recent developments of SMFPs based on enzyme activity, plasma membranes, reactive oxygen species, reactive sulfur species, microenvironments and others during cell apoptosis. In particular, we generalize the mechanisms commonly used to design SMFPs for studying apoptosis. In addition, we discuss the limitations of reported probes, and emphasize the potential challenges and prospects in the future. We believe that this review will provide a comprehensive summary and challenging direction for the development of SMFPs in apoptosis related fields.

ClO- Induced Dual-Excitation Fluorescent Probes Responding to Diverse Testing Modes with Ratio Methodology

Single-excitation ratio fluorescent probes have enabled the output signal with high signal-to-noise ratio, but are still plagued with technique challenges, including signal distortion and limited application scenario. Herein, a dual-excitation near-infrared (NIR) fluorescent probe P1 of coumarin derivatives is constructed, showing high signal output ability in the visible region and high tissue penetration depth ability in the NIR region. As NIR probe P1 selectively recognizes ClO^-, the emission signal in the visible region (480 nm) of P1 is enhanced during the recognition process. Meanwhile, the NIR emission (830 nm) of the conjugated system is weakened, finally realizing that ClO^- triggered the dual-excitation (720/400 nm) ratio fluorescence signal detection and monitoring. The signal of detection in vitro has high responsiveness. Meanwhile, in the process of NIR monitoring in vivo, positive contrast imaging of fluorescence is constructed, which can accurately monitor ClO^- changes over time. The current dual-excitation fluorescence-based data calibration and/or comparison method improves the application of the traditional single-excitation ratio fluorescence strategy and provide innovative detection tools for accurate measurement of fluorescence detection, with detection/monitoring modes suitable for different physiological environments.

Construction of Genetically Encoded Biosensors to Monitor Subcellular Compartment-Specific Glutathione Response to Chemotherapeutic Drugs in Acute Myeloid Leukemia Cells

Glutathione (GSH), the constituent of the redox buffer system, is a scavenger of reactive oxygen species (ROS), and its ratio to oxidized glutathione (GSSG) is a key indicator of oxidative stress in the cell. Acute myeloid leukemia (AML) is a highly aggressive hematopoietic malignancy characterized by aberrant levels of reduced and oxidized GSH due to oxidative stress. Therefore, the real-time, dynamic, and highly sensitive detection of GSH/GSSG in AML cells is of great interest for the clinical diagnosis and treatment of leukemia. The application of genetically encoded sensors to monitor GSH/GSSG levels in AML cells is not explored, and the underlying mechanism of how the drugs affect GSH/GSSG dynamics remains unclear. In this study, we developed subcellular compartment-specific sensors to monitor GSH/GSSG combined with high-resolution fluorescence microscopy that provides insights into basal GSH/GSSG levels in the cytosol, mitochondria, nucleus, and endoplasmic reticulum of AML cells, in a decreasing order, revealing substantial heterogeneity of GSH/GSSG level dynamics in different subcellular compartments. Further, we investigated the response of GSH/GSSG ratio in AML cells caused by Prussian blue and Fe3O4 nanoparticles, separately and in combination with cytarabine, pointing to steep gradients. Moreover, cytarabine and doxorubicin downregulated the GSH/GSSG levels in different subcellular compartments. Similarly, live-cell imaging showed a compartment-specific decrease in response to various drugs, such as CB-839, parthenolide (PTL), and piperlongumine (PLM). The enzymatic activity assay revealed the mechanism underlying fluctuations in GSH/GSSG levels in different subcellular compartments mediated by these drugs in the GSH metabolic pathway, suggesting some potential therapeutic targets in AML cells.

Self-Cascade Nanoenzyme of Cupric Oxide Nanoparticles (CuO NPs) Induced in Situ Catalysis Formation of Polyelectrolyte as Template for the Synthesis of Near-Infrared Fluorescent Silver Nanoclusters and the Application in Glutathione Detection and Bioimaging

In this work, near-infrared fluorescent silver nanoclusters (Ag NCs) were prepared based on the in situ formed poly methacrylic acid (PMAA) as the template and stabilizer, which is synthesized by methacrylic acid (MAA) and hydroxyl radical (·OH) that is generated by the cascade nanoenzyme reaction of cupric oxide nanoparticles (CuO NPs). CuO NPs possess the intrinsic glutathione-like (GPx-like) and peroxidase-like (POD-like) activities, which can catalyze glutathione (GSH) and O₂ to produce hydrogen peroxide (H₂O₂), and then transform into ·OH. The fluorescence intensity of Ag NCs decreases with the addition of GSH, because the -SH can easily anchor on the surface, resulting in the PMAA leaving the Ag NCs, and the coeffect of GSH and PMAA results in the aggregation to form larger Ag NPs. A good linear relationship between the fluorescence quenching rate and the GSH concentration was found in the range 0.01-40 μM with the detection limit 8.0 nM. The Ag NCs can be applied in the detection of GSH in the serum, as well as bioimaging of endogenous and exogenous GSH in cells with high sensitivity. Moreover, the normal and cancer cells can be distinguished through bioimaging because of the different GSH levels. The new method for the preparation of biocompatible nanoprobe based on the nanozyme tandem catalysis and the in situ formed template can avoid the direct usage of polymers or protein templates that hinder preparation and separation, providing a reliable approach for the synthesis, biosensing, and bioimaging of nanoclusters.

Patented AIE materials for biomedical applications

In recent years aggregation induced emission (AIE) concept has attracted researcher's interest worldwide. Several organic building blocks are developed as AIE materials. This chapter discusses the patented AIE material and their utilization related in biological, medicinal and biotechnology fields. It is demonstrated that AIE chromophores such as tetraphenylethylene (TPE) as well as other AIE building blocks became important fluorescent emissive bioactive materials. Such emissive materials are widely employed as bioprobes for the detection of mitochondria, cellular imaging and tracking, protein carrier detection of S-phase DNA, detection of d-glucose, visualization of cancer treatment, drug screening, image-guided therapy, bacterial imaging, photodynamic therapy and drug screening. Such AIE materials upon imaging in cellular environment displays significant enhancement of fluorescence emission. Such patented AIE chromophores has a great potential for bioimaging and biomedical applications. In this chapter we compile some patented representative examples to explore their bioimaging/medicinal imaging applications since lot of new inventions are reported every day.

Fluorescent Probes for Live Cell Thiol Detection

Thiols play vital and irreplaceable roles in the biological system. Abnormality of thiol levels has been linked with various diseases and biological disorders. Thiols are known to distribute unevenly and change dynamically in the biological system. Methods that can determine thiols' concentration and distribution in live cells are in high demand. In the last two decades, fluorescent probes have emerged as a powerful tool for achieving that goal for the simplicity, high sensitivity, and capability of visualizing the analytes in live cells in a non-invasive way. They also enable the determination of intracellular distribution and dynamitic movement of thiols in the intact native environments. This review focuses on some of the major strategies/mechanisms being used for detecting GSH, Cys/Hcy, and other thiols in live cells via fluorescent probes, and how they are applied at the cellular and subcellular levels. The sensing mechanisms (for GSH and Cys/Hcy) and bio-applications of the probes are illustrated followed by a summary of probes for selectively detecting cellular and subcellular thiols.

Silicon nanoparticles-based ratiometric fluorescence platform: Real-time visual sensing to ciprofloxacin and Cu2

In this work, a silicon nanoparticles (Si NPs)-based ratiometric fluorescence sensing platform was conveniently fabricated by simply mixing fluorescent Si NPs as co-ligands and reference signal with lanthanide metal ion Eu³⁺ as response signal. The introduction of ciprofloxacin (CIP) remarkably turned on the characteristic fluorescence of Eu³⁺ at 590 nm and 619 nm through the "antenna effect". At the same time, the blue emission of Si NPs at 445 nm kept comparatively stable. A good linear relationship between the ratio fluorescence intensity and CIP concentration in the range of 0.211-132.4 μM with a limit of detection (LOD) of 89 nM was obtained. In the presence of Cu²⁺, the fluorescence emission of Eu³⁺ was sharply turned off because of the stronger coordination ability of Cu²⁺ with CIP, which guaranteed the sequential detection of Cu²⁺. Meanwhile, the distinct fluorescent color change from bright blue to red, then back to blue, enabled naked-eye visual detection of CIP and Cu²⁺ in the solution phase and on paper-based test strip, and was successfully applied to determine the levels of CIP in complicated food samples with high sensitivity.

Fluorescent probe based on N-doped carbon dots for the detection of intracellular pH and glutathione

Carbon dots (CDs) as fluorescent probes have been widely exploited to detect biomarkers, however, tedious surface modification of CDs is generally required to achieve a relatively good detection ability. Here, we synthesized N-doped carbon dots (N-CDs) from triethylenetetramine (TETA) and m-phenylenediamine (m-PD) using a one-step hydrothermal method. When the pH increases from 3 to 11, the fluorescence intensity of the N-CDs gradually decreases. Furthermore, it displays a linear response to the physiological pH range of 5-8. Au³⁺ is reduced by amino groups on the surface of N-CDs to generate gold nanoparticles (AuNPs), causing fluorescence quenching of the N-CDs. If glutathione (GSH) is then added, the fluorescence of the N-CDs is recovered. The fluorescence intensity of the N-CDs is linearly correlated with the GSH concentration in the range of 50-400 μM with a limit of detection (LOD) of 7.83 μM. The fluorescence probe was used to distinguish cancer cells from normal cells using pH and to evaluate intracellular GSH. This work expands the application of CDs in multicomponent detection and provides a facile fluorescent probe for the detection of intracellular pH and GSH.

Mitochondria-targeted drug delivery in cancers

Mitochondria are considered one of the most important subcellular organelles for targeting and delivering drugs because mitochondria are the main location for various cellular functions and energy (i.e., ATP) production, and mitochondrial dysfunctions and malfunctions cause diverse diseases such as neurodegenerative disorders, cardiovascular disorders, metabolic disorders, and cancers. In particular, unique mitochondrial characteristics (e.g., negatively polarized membrane potential, alkaline pH, high reactive oxygen species level, high glutathione level, high temperature, and paradoxical mitochondrial dynamics) in pathological cancers have been used as targets, signals, triggers, or driving forces for specific sensing/diagnosing/imaging of characteristic changes in mitochondria, targeted drug delivery on mitochondria, targeted drug delivery/accumulation into mitochondria, or stimuli-triggered drug release in mitochondria. In this review, we describe the distinctive structures, functions, and physiological properties of cancer mitochondria and discuss recent technologies of mitochondria-specific "key characteristic" sensing systems, mitochondria-targeted "drug delivery" systems, and mitochondrial stimuli-specific "drug release" systems as well as their strengths and weaknesses.

Real-time monitoring of glutathione in living cells using genetically encoded FRET-based ratiometric nanosensor

Reduced glutathione (GSH) level inside the cell is a critical determinant for cell viability. The level of GSH varies across the cells, tissues and environmental conditions. However, our current understanding of physiological and pathological GSH changes at high spatial and temporal resolution is limited due to non-availability of practicable GSH-detection methods. In order to measure GSH at real-time, a ratiometric genetically encoded nanosensor was developed using fluorescent proteins and fluorescence resonance energy transfer (FRET) approach. The construction of the sensor involved the introduction of GSH binding protein (YliB) as a sensory domain between cyan fluorescent protein (CFP; FRET donor) and yellow fluorescent protein (YFP; FRET acceptor). The developed sensor, named as FLIP-G (Fluorescence Indicator Protein for Glutathione) was able to measure the GSH level under in vitro and in vivo conditions. When the purified FLIP-G was titrated with different concentrations of GSH, the FRET ratio increased with increase in GSH-concentration. The sensor was found to be specific for GSH and also stable to changes in pH. Moreover, in live bacterial cells, the constructed sensor enabled the real-time quantification of cytosolic GSH that is controlled by the oxidative stress level. When expressed in yeast cells, FRET ratio increased with the external supply of GSH to living cells. Therefore, as a valuable tool, the developed FLIP-G can monitor GSH level in living cells and also help in gaining new insights into GSH metabolism.

Quinolone-based fluorescent probes for distinguished detection of Cys and GSH through different fluorescence channels

Dual-channel discrimination of Cys and GSH using a red fluorescent probe.

Light-activated "cycle-reversible intramolecular charge transfer" fluorescent probe: monitoring of pHi trace change induced by UV light in programmed cell death

Under the synergistic effects of protonation and deprotonation, a light-activated fluorescent probe (UV-SP) exhibited "cycle-reversible intramolecular charge transfer (ICT)" for different pH after activation by UV light, resulting in emission of multiple ratio fluorescent signals (FI563/FI595 and FI664/FI595). Based on these kinds of response signals, UV-SP can specifically monitor the cycle-reversible trace change of intracellular pH caused by UV radiation. More importantly, according to the stable and invariant multiple ratio fluorescent signals, UV-SP can sort cells entering programmed death.

Regulating glutathione-responsiveness of naphthalimide-based fluorescent probes by an oxidation strategy

Two naphthalimide-based fluorescent probes containing a thiomorpholine (Np-NS) or a sulfoxide-morpholine (Np-NSO) component are reported. The morpholine unit of non-fluorescent Np-NS and Np-NSO can transform into sulphone-morpholine and be accompanied by blue fluorescence upon oxidative stress, ascribed to the formation of sulphone-morpholine on probes. This sensing behavior displays that they can selectively respond to glutathione to generate a green emission by a sulfonamide-based detection moiety both in vitro and in living cells. Interestingly, the different oxidation states of a sulphur atom on a thiomorpholine ring can be utilized to regulate responsiveness of these probes towards glutathione. Such an oxidation strategy would provide a possibility for enhancing the response rate.

Fluorescence detection of intracellular pH changes in the mitochondria-associated process of mitophagy using a hemicyanine-based fluorescent probe

Intracellular pH behaves as a vital parameter in the physiological and pathological processes. Novel small molecule probes for precise and dynamic monitoring of pH fluctuations in cellular physiological processes are still highly required. Herein, we present a hemicyanine-based probe (HcPH) detection of the pH changes during the intracellular process of mitochondria-associated autophagy. HcP-H exhibits highly reversible and ratiometric fluorescence detection of pH variation due to the deprotonation/protonation process, showing orange fluorescence (λ_em = 557 nm) in basic media (pH 8.0) and green fluorescence (λ_em = 530 nm) in acidic media (pH 6.2), respectively. Organelle localization experiment in HeLa cells demonstrates that this probe could selectively accumulate in mitochondria, showing almost overlap with that of Mito-Tracker Green FM. More importantly, Fluorescence imaging of HcP-H in HeLa cells subjected to the nutrient deprivation has demonstrated that this probe could monitor the intracellular pH changes in the mitochondria-associated process of mitophagy. It is clearly confirmed that HcP-H would serve as a promising fluorescent probe for tracing mitophagy in living cells.

A novel turn-on fluorescent probe for selective sensing and imaging of glutathione in live cells and organisms

A novel 1-oxo-1H-phenalene-2,3-dicarbonitrile (OPD)-based fluorescent probe was developed to sense and image GSH in HeLa cells, different imatinib-resistant K562 cells, D. magna and zebrafish embryos.

Thiol Specific and Mitochondria Selective Fluorogenic Benzofurazan Sulfide for Live Cell Nonprotein Thiol Imaging and Quantification in Mitochondria

Cellular thiols are divided into two major categories: nonprotein thiols (NPSH) and protein thiols (PSH). Thiols are unevenly distributed inside the cell and compartmentalized in subcellular structures. Most of our knowledge on functions/dysfunctions of cellular/subcellular thiols is based on the quantification of cellular/subcellular thiols through homogenization of cellular/subcellular structures followed by a thiol quantification method. We would like to report a thiol-specific mitochondria-selective fluorogenic benzofurazan sulfide {7,7'-thiobis( N-rhodamine-benzo[c][1,2,5]oxadiazole-4-sulfonamide) (TBROS)} that can effectively image and quantify live cell NPSH in mitochondria through fluorescence intensity. Limited methods are available for imaging thiols in mitochondria in live cells especially in a quantitative manner. The thiol specificity of TBROS was demonstrated by its ability to react with thiols and inability to react with biologically relevant nucleophilic functional groups other than thiols. TBROS, with minimal fluorescence, formed strong fluorescent thiol adducts (λ_ex = 550 nm, λ_em = 580 nm) when reacting with NPSH confirming its fluorogenicity. TBROS failed to react with PSH from bovine serum albumin and cell homogenate proteins. The high mitochondrial thiol selectivity of TBROS was achieved by its mitochondria targeting structure and its higher reaction rate with NPSH at mitochondrial pH. Imaging of mitochondrial NPSH in live cells was confirmed by two colocalization methods and use of a thiol-depleting reagent. TBROS effectively imaged NPSH changes in a quantitative manner in mitochondria in live cells. The reagent will be a useful tool in exploring physiological and pathological roles of mitochondrial thiols.

A mitochondria-targeting fluorescent probe for the selective detection of glutathione in living cells

We report a fluorescent probe for the selective detection of mitochondrial glutathione (GSH). The probe, containing triphenylphosphine as a mitochondrial targeting group, exhibited ratiometric and selective detection of GSH over Cys/Hcy. The probe was used for imaging mitochondrial GSH in living HeLa cells.

Fast and Selective Two-Stage Ratiometric Fluorescent Probes for Imaging of Glutathione in Living Cells

Two fluorescent, m-nitrophenol-substituted difluoroboron dipyrromethene dyes have been designed by nucleophilic substitution reaction of 3,5-dichloro-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY). Nonsymmetric and symmetric probes, that is. BODIPY 1 (with one nitrophenol group at the position 3) and BODIPY 2 (with two nitrophenol groups at the positions 3 and 5) were applied to ratiometric fluorescent glutathione detection. The detection is based on the two-step nucleophilic aromatic substitution of the nitrophenol groups of the probes by glutathione in buffer solution containing CTAB. In the first stage, probe 1 showed ratiometric fluorescent color change from green (λ_em = 530 nm) to yellow (λ_em = 561 nm) because of monosubstitution with glutathione (I_561nm/I_530nm). Addition of excess glutathione caused the second stage of ratiometric fluorescent color change from yellow to reddish orange (λ_em = 596 nm, I_596nm/I_561nm) due to disubstitution with glutathione. Therefore, different concentration ranges of glutathione (from less to excess) could be rapidly detected by the two-stage ratiometric fluorescent probe 1 in 5 min. While, probe 2 shows single-stage ratiometric fluorescent detection to GSH (from green to reddish orange, I_596nm/I_535nm). Probes 1 and 2 exhibit excellent properties with sensitive, specific colorimetric response and ratiometric fluorescent response to glutathione over other sulfur nucleophiles. Application to cellular ratiometric fluorescence imaging indicated that the probes were highly responsive to intracellular glutathione.

Development of dual-fluorescence cell-based biosensors for detecting the influence of environmental factors on nanoparticle toxicity

With the expanding use of engineered nanoparticles (NPs), development of a high-throughput, sensitive method for evaluating NP safety is important. In this study, we developed cell-based biosensors to efficiently and conveniently monitor NP toxicity. The biosensor cells were obtained by transiently transfecting human cells with biosensor plasmids containing a mCherry gene regulated by an inducible promoter [an activator protein 1 (AP-1) promoter, an interleukin 8 (IL8) promoter, or a B cell translocation gene 2 (BTG2) promoter], with an enhanced green-fluorescent protein gene driven by the cytomegalovirus promoter as the internal control. After optimizing flow cytometric analysis, these dual-fluorescence cell-based biosensors were capable of accurately and rapidly detecting NP toxicity. We found that the responses of AP-1, BTG2, and IL8 biosensors in assessing the toxicity of silver nanoparticles (Ag NPs) showed good dose-related increases after exposure to Ag NPs and were consistent with data acquired by conventional assays, such as western blot, real-time polymerase chain reaction, and immunofluorescence. Further investigation of the effects of environmental factors on Ag NP toxicity revealed that aging in water, co-exposure with fulvic acid, and irradiation with ultraviolet A light could affect Ag NP-induced biosensor responses. These results indicated that these novel dual-fluorescence biosensors can be applied to accurately and sensitively monitor NP toxicity.

A primary SERS-active interconnected Si-nanocore network for biomolecule detection with plasmonic nanosatellites as a secondary boosting mechanism

We report in this study, the development of a polymorphic biosensitive Si nanocore superstructure as a SERS biosensing platform.

Live cell monitoring of glycine betaine by FRET-based genetically encoded nanosensor

Glycine betaine (GB) is one of the key compatible solutes that accumulate in the cell at exceedingly high level under the conditions of high salinity. It plays a crucial role in the maintenance of osmolarity of the cell without affecting the physiological processes. Analysis of stress-induced physiological conditions in living cells, therefore, requires real-time monitoring of cellular GB level. Glycine Betaine Optical Sensor (GBOS), a genetically-encoded FRET-based nanosensor developed in this study, allows the real-time monitoring of GB levels inside living cells. This nanosensor has been developed by sandwiching GB binding protein (ProX) between the Förster resonance energy transfer (FRET) pair, the cyan fluorescent protein (CFP) and yellow fluorescent protein (YFP). Conformational change in ProX, which was used as sensory domain, reported the change in the level of this compatible solute in in vitro and in vivo conditions. Binding of the GB to the sensory domain fetches close to both the fluorescent moieties that result in the form of increased FRET ratio. So, any change in the concentration of GB is correlated with change in FRET ratio. This sensor also reported the GB cellular dynamics in real-time in Escherichia coli cells after the addition of its precursor, choline. The GBOS was also expressed in yeast and mammalian cells to monitor the intracellular GB. Therefore, the GBOS represents a unique FRET-based nanosensor which allows the non-invasive ratiometric analysis of the GB in living cells.